Fat Metal is heavier than heavy metal, or rotund metal, or even big-boned metal. It is metal that is just too heavy for any other description than straight up fat. It draws inspiration from all forms of music; anything that makes the riffs heavier than ever thought possible. In essence, it is like an aural face push: it hits you hard, and is so heavy that it leaves a feeling of inferiority with you at the same time.
Person 2: Like fat heavy, or heavy metal heavy?
Person 1: Both. They're fat, and also play fat metal.